In recent years, improvements in performance of computer are remarkable so that computer-computer communications are implemented usually in ordinary home. Furthermore, internet is developed in world-wide scale so that circumstances in communication are changed violently.
Under these circumstances, information is required instantly by one who is concerned with communications. Upon demand at such request, optical-fiber telecommunication system by which a large amount of various information can be transmitted at high-speed is utilized as the media.
Recently, it is required to transmit at real time not only character information, but also a variety of information such as still image, dynamic image and speech data, so that high-capacity optical-fiber telecommunication system becomes developed.
Moreover, in order to transmit data through an optical-fiber telecommunication system over great distances, it is required to amplify lightwave signals in the course of optical-fiber line. In this respect, introduction of erbium doped optical-fiber amplification (EDFA) is made also as means for light amplification.
When EDFA system is applied as means for light amplification, wavelength division multiplex is performed within a wavelength band of EDFA (about 30 nm in the existing state) to make mass-storage of data, so that the above described transmission system is proceeding toward practical utilization.
In this system, a number of light sources each having a specified wavelength are required. In the case where a plurality of light sources are used, there are disadvantages of high cost, upsizing of equipment, high consumption of energy and the like, and hence, it is desired to eliminate these disadvantages.
Accordingly, it is desired to realize a wavelength tunable laser in which a wavelength can be tuned arbitrarily within the wavelength band of EDFA, i.e., that of about 30 nm, so that the wavelength can be specified to a predetermined wavelength.
Wavelength tunable lasers are classified broadly into semiconductor monolithic types and external mirror types. As to the semiconductor monolithic type wavelength tunable laser, no practical unit exists which can tune wavelength within a range of wavelength as wide as about 30 nm corresponding to that of EDFA.
On the other hand, the disadvantage of the external mirror type is that a mechanism for rotating a grating type reflector becomes upsizing. Where such grating type reflector is used as the external mirror so that tuning is done by changing the incident angle of light, wherein a manner for rotating the grating reflector by the use of a motor and the like manner is required.